Have to re-upload images to update new post_thumbnail sizes??

I’ve set up post thumbnails in my functions.php file like so:

// post thumbnails
add_theme_support('post-thumbnails');
set_post_thumbnail_size(50, 50, true);
add_image_size('info', 160, 9999);
add_image_size('img-post', 575, 600);
add_image_size('img-work', 900, 595);

I then call them on my templates like this:

<?php query_posts('post_type=work-architecture&posts_per_page=-1'); ?>
	<?php if(have_posts()) : ?>
		<?php while(have_posts()) : the_post(); ?>
			<?php the_post_thumbnail('img-work'); ?>
		<?php endwhile; ?>
	<?php endif; ?>
<?php wp_reset_query(); ?>

Originally I had the boolean ‘crop’ parameter set to ‘true’ on ‘img-work’, so all of those images were being cropped.

I have since removed this parameter as I do not wish for the images to be cropped, but this does not update when viewing my website through the front end.

It seems that the only way I can update the images is by removing them from the media library and re-uploading them.

What’s most confusing is how the images in the media library are not cropped, so I cannot possibly think of why they are showing up as cropped on the frontend??

Do I somehow need to clear the cache in WordPress?

Matt.